Polyurethane decomposition mechanism

Similar arguments explaining the phase separation were employed by Chou et al. [44]. The dynamics of phase separation was observed using an optical microscope during the course of polyurethane-unsaturated polyester IPN formation at different temperature. Chou et al. suggested that an interconnected phase formed through the spinodal decomposition mechanism developed quickly and was followed by the coalescence of the periodic phase to form a droplet/matrix type of morphology. The secondary phase separation occurred within both the droplet and the matrix phases. Chou et al. did not explain, however, why secondary phase separation occurred. 

Fig. 7.5. Peroxide crosslinking mechanism of an elastomer. This mode of decomposition occurs in neutral or alkaline conditions, as normally prevails during the crosslinking of millable polyurethane rubber. If an acidic phase predominates then little or no crosslinking occurs as a different peroxide decomposition mechanism takes place.

Hydrolysis can be defined as the decomposition of a compound by reaction with water, the water taking part in the reaction. The effect is enhanced by the presence of either acids or alkalis. The chemistry of polyurethanes leads to the probability of hydrolytic attack. The mechanism is illustrated in Figure 2.41. 

Even a single polymer can have a composite structure. Here, the phase geometry and mechanical properties are considered for polymers that separate into two amorphous phases. Block copolymers usually have sufficient block lengths to allow micron-scale phase separation. Later on, we have considered smaller scale microstructures caused by the spinodal decomposition of polyurethanes. Semi-crystalline polymers will be considered in Section 4.6. 

Ashida et burnt only 0.3 g of polyurethane foam in an air stream of 150 cm /min and found a maximum of HCN generation at about 550 °C. Jellinek / aj Uy 45 proposed the kinetics and mechanism ofHCN production from polyurethanes and polyimides. They found that HCN which evolved at lower temperatures underwent oxidation and thermal decomposition between 800 and 1000 C, whilst its total amount remained constant due to the additional HCN production from the char residue even above 800 °C. 

While thermoplastic polymers soften at Tg, and if semicrystalline, melt at T, cross-linked polymers do not melt and flow (Fig. 5.2c ). Lightly cross-linked polymers soften as the temperature exceeds Tg, but they remain rubbery solids until the polymer decomposes. Highly cross-linked polymers often do not even soften and retain a high modulus until reaching the decomposition temperature. Thermoset resins, like unsaturated polyester, epoxy, and polyurethanes, have varied levels of cross-linking. However, thermoplastic resins can be modified to contain few cross-links lightly cross-linked polyethylene (XLPE) often improves the mechanical properties of rotomolded parts. 

Kinetic studies have been made on the thermal decomposition of a poly(oxypropylene)triol-toluene di-isocyanate copolymer foam. Following a diffusion rate-controlled step, the cellular structure collapses to a viscous liquid and degradation then occurs on a random scission basis. Products of degradation of A-monosubstituted and A A-disubstituted polyurethanes have been analysed by direct pyrolysis in the ion source of a mass spectrometer. The mono-substituted polymers depolymerize quantitatively to di-isocyanates and diols, whereas the disubstituted materials decompose selectively to secondary amines, olefins, and carbon dioxide. The behaviour of the monosubstituted polymers has been confirmed in an i.r. study of the degradation of model compounds. A study of the thermal degradation in vacuum of polyurethanes prepared from butanediol, methylene bis(4-phenylisocyanate), and hexanedioic acid-ethylene glycol-propylene glycol polyesters has been reported and reaction mechanisms proposed. ... 

In isothermal determinations the shapes of CL intensity on time are individualize for different oxidation mechanisms involving specific free radicals. If polyolefins generate sigmoid form of CL curves [04G1], for polyamides [03C1] or polyurethanes [07J1] an early peak is recorded in CL intensity measurements followed by parabolic decrease. In any case, the CL intensity values on the first five minutes of measurements define the initial state of oxidation rather than the instant decomposition process. Either isothermal CL measurements or nonisothermal CL investigations are reliable and accurate tools for... 

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